The invention relates generally to the field of polyphase motor excitation and control systems and particularly to systems for multistate modulation excitation of three terminal three phase motors.
In AC motors and brushless dc motors, fixed primary windings, usually called stator windings, are distributed about the axis of a rotor, which in turn is coupled to the motor shaft. Alternating currents of different phases of the same frequency are supplied to these stator windings to create a sinusoidally distributed rotating field in the machine air gap. Three phase motor construction and excitation is commonly used with the stator windings in either of two conventional configurations designated "wye" or "delta". The motor may be of the synchronous type wherein the rotor contains permanent magnets or separately electrified windings or the motor may be of the asynchronous type wherein the rotor is comprised of secondary conductors arranged in a shorted turn configuration, in which case the motor is referred to as an "induction" motor.
Historically, AC motors and particularly induction motors, have been relegated primarily to fixed speed applications. However, with advances in semiconductor switching technology, it has become feasible to provide reliable low cost alternating current sources of variable frequency and amplitude, thus providing an opportunity for enhanced control of the AC motor at varying speeds and loads. Prior art switching systems, discussed in more detail below, have been designed that create a pulse waveform which approximates a sinusoidal waveform across each motor terminal pair.
For applications with tight requirements on ripple torques (especially at low speed), the currents in the stator windings must be kept very sinusoidal. High efficiency polyphase switching systems employed to create sinusoidal currents ordinarily use some form of pulse width modulation. In the prior art, these systems broke into two classes: one based on current excitation and the other on voltage excitation. The current excitation class involved what is commonly referred to as the current mode inverter technology. A single controlled "link" current was electronically rapidly "reconnected", for varying time intervals, to a different pair of motor terminals so as to create balanced pulse-width modulated current signals into each terminal. Although the harmonic current energy was high in these systems, its spectrum could be controlled to achieve adequate performance in systems with modest ripple torque specifications.
The other multiphase prior art systems which attempted to satisfy balanced sinusoidal current requirements, utilized a dc voltage link with individual two state modulators, typically pulsewidth modulators, creating balanced but variable voltage excitation to the motor terminals. In this configuration, current feedback was conventionally incorporated by sensing an appropriate common current measure, for example, the multiphase full wave rectified terminal currents. The current error signal from a dc reference would then simultaneously modify the amplitudes of all three modulators.
Direct measurement of an individual modulator's instantaneous output current (terminal current) with individual feedback to control that modulator's command has many potential advantages such as high bandwidth, precise phase and amplitude control and is the dominant technique used in high performance two phase motor controller designs (principally brushless dc motors). These two phase motors were driven either from a single dc supply utilizing four motor terminals or from a dual supply (three terminals) with positive and negative supply voltages and a common ground. Application of prior art individual terminal direct current feedback techniques to n phase, n terminal motors (n.gtoreq.3) from a single dc supply presents problems due to the Kirchoff current constraint to which no optimum solution exists in the prior art.